Isomerization of olefins in stripping



Oct. 24, 1961 A. D. BAARS ETAL ISOMERIZATION 0F OLEFINS IN STRIPPING Filed Jan. 19, 1960 A W n u m 4 a. H a 1 u T m INVENTORSI ARIE D. BAARS WILLIAM J. WOLFSON THEIR ATTORNEY tates This invention relates to an improvement in petroleum refining Operation through better utilization of fluid catalytic cracking units. More specifically, it relates to the upgrading of olefinic C and/ or C fractions by making use of existing catalytic cracking stripping capacity. Thus it allows refinery operations to be improved with little or no capital expenditure.

In modern petroleum refining operations catalytic cracking plays a most important role since it is responsible for the major part of the gasoline produced. Most refineries therefore have at least one catalytic cracking unit and some have several. While there are other types of catalytic cracking units most of them are so called fluid catalytic cracking units in which a powdered silica-alumina containing catalyst is continuously circulated through a reaction zone, a stripping zone, and a regeneration zone.

While gasoline is normally the refiners chief and most important product and the catalytic cracking unit is operated under conditions to maximize the yield of gasoline of suit-able quality, there are in the usual refinery small streams of other materials of lesser value which should be utilized as profitably as possible. Twosuch materials are the so called BB (butane-butylene) and PA (pentaneamylenes) fractions. These fractions are often quite olefinic. Those obtained directly from the catalytic cracking operation contain the various olefin isomers in approximately the concentrations corresponding to the thermodynamic equilibrium at the cracking temperature, whereas those derived from thermal operations or from a further processing step do not. For example the PA fraction generally contains sizeable amounts of pentenes having a branched chain. These are very desirable hydrocarbons and it is desired to separate them. This is done for instance by absorbing them in sulfuric acid. This is not efficient, however, since the 3-methyl butene-l, although having a branched structure, contains its ethylenic bond in the terminal position and is not absorbed with the other tertiary pentenes. It would be desirable to isomerize the raffinate from the sulfuric acid treatment to allow recovery of this component but this could not be justified economically primarily because of the capital requirements for an isomerization unit.

A similar situation is found with the BB fraction. This fraction is normally used in the production of alkylate. However, the quality of the alkylate produced is largly influenced by the nature of the butylenes. Thus, the F 44% octane number (the performance number of the material after the addition of 4 /2 ml. tetraethyl lead per US. gallon) of the alkylate from isobutene and isobutane is 152, of the alkylate from isobutane and butene-Z, 155, and of the alkylate from isobutane and butene-l, 126. From these numbers it would be seen that the BB fraction going to alkylation preferably contains little butene-l; in other words, the isobutene plus butene-Z to butene-l ratio is preferably as high as possible.

It is known that butene-l can be isomerized to butene-Z up to limits set by the thermodynamic equilibrium; see for example French Patent No. 913,756. However, here again the improvement possible by decreasing the content of butene-l from say twice its equilibrium concentration to its equilibrium concentration is entirely insufficient to justify an isomerization unit.

It has been found that in either or both of these cases atent the improvement can be obtained with little or no capital expenditure and without altering or harming the normal operation of the catalytic cracking unit by utilizing the capacity available in the stripping operation.

The process of the invention is particularly suitable for the upgrading in BB fractions and PA fractions as mentioned above, whenever these fractions contain olefins having a terminal double bond in a concentration above that corresponding to the equilibrium. If desired the material may contain both butylenes and amylenes. It is particularly suitable for the treatment of the BB fractions derived from thermal cracking or reforming treatments. Such fractions generally have a butene-Z plus isobutene to butene-l ratio of less than 2. With the use of the process according to the invention it was found possible to increase this ratio considerably e.g., to values of from 3 to 6 (depending upon the conditions). In addition to the mentioned isomerization which involves a shift of the position of the double bond, some side reactions occur to a limited extent e.g., isomerization of a carbon skeleton and hydrogenation of double bonds by hydrogen transfer. Actual losses due to decomposition etc. are however negligibly small.

The catalytic cracking operation, as mentioned above, is carried out for the production of gasoline under the normal conditions. Although these conditions may vary somewhat due to different feed stocks, activity of the catalyst, etc. they are all known and need not be further described, except to mention that the temperature in the stripping zone is almost completely a function of the temperature of the catalyst feed to the stripping zone from the reactor and normally lies between about 450 and 540 C.

As mentioned above a characteristic of the fluidized catalytic cracking process is that the catalyst is continuously circulated from the reaction zone through a stripping zone to a regeneration zone and back to the reac' tion zone. In some units the stripping zone is a partitioned zone within the reactor vessel. In other units the stripping is effected in a separate vessel. The catalyst is preferably stripped while it flows in the form of a suspension in the stripping agent through an elongated riser after which it is subsequently stripped in a conventional (dense) fluidized bed. Such a riser will usually have a length to the ratio of at least 4 to 1, for example 30 to 1. When the process is applied in this type of an arrangement the mixture upon leaving the riser is preferably more or less completely separated into stripped catalyst, on the one hand, and steam with hydrocarbon vapors on the other and the catalyst is then further stripped in the fluidized state with steam. This embodiment, in which the isomerization is carried out in the first stage of stripping and subsequent stripping is effected in a second stage, is preferred to that in which the feed to be isomerized is injected into a fluidized catalyst mass which mass is stripped in a single stage since in the former process the catalyst entering the regeneration zone has the least possible hydrocarbon con tent. In this connection it should be noted when operating as above described it has been found that the hydrocarbon content of the spent and stripped catalyst which is led to the regeneration zone is not noticeably affected by the presence in the first stripping zone of the o-lefinic hydrocarbons added. On the contrary, the injection of the olefin fraction makes it often possible to effect an appreciable reduction in the steam requirements for stripping and this is an economic advantage.

The invention will be further described with reference to the accompanying drawing which shows a diagram of a catalytic cracking reactor with a built in stripping zone A separated from the reaction zone B by the partition 1. Above the partition the reaction zone and stripping zone are in open communication. The levels in the fluidized beds in zones A and B are below the top of the partition 1.

In this arrangement the gases and vapors in the stripping zone are discharged from the reaction zone together with the cracked products, that is through line 2. The oil is supplied through line 3 together with the regenerated catalyst. Spent catalyst flows through line 4 from the reaction zone B to the riser 5 and is passed in the form of a suspension through the riser by means of steam supplied by way of line 6, stripping already occurring to a substantial extent in the riser. The catalyst then finds its way to the fluidized bed in zone A. The bed level may be below or above the top of the riser but is preferably below. The catalyst is then further stripped in the fluidized bed with steam supplied through line 7. The stripped catalyst leaves the stripping zone through line 8 for regeneration in the regenerator (not shown).

According to the invention an olefin or olefinic fraction of C -C hydrocarbons, of which those having a terminal double bond are present in greater than the equilibrium concentration, is supplied through line 9.

EXAMPLE 1 In a commercial unit 23 to 24 tons of spent catalyst were passed per minute from the catalytic reactor to a stripper riser (length 25 meters, diameter 0.9 meter) in which it was stripped with steam. For this purpose 257 tons of steam were used daily when operating catalytic cracking unit in conventional manner with no olefin injection. The catalyst was subsequently stripped in a fluidized bed in a stripping zone with an additional amount of 60 tons per day of steam.

The temperature in the riser was 465-475 C.

In a first experiment according to the invention an amount of 100 tons a day of an olefinic C fraction was supplied to the of the stripper riser and the amount of steam supplied was reduced to 215 tons per day. The C traction was separated from the gaseous efliuent. It was found that there had been practically no loss of C hydrocarbons. The composition of the starting material and the C fractions are shown as following Table I.

It should be mentioned that replacing a part of the steam by a C fraction did not noticeably aifect the operation of the reactor, stripper or the regenerator and in particular did not afi-ect the amount or composition of the cracked products obtained in the cracking operation.

EXAMPLE 2 In a second experiment in the same unit as in Example 1 a quantity of 200 tons of a BB fraction was supplied daily to the stripper riser and the quantity of steam supplied to the riser was reduced to 161 tons a day. Table 2 below shows the composition of the starting material and the recovered C fraction. Practically no loss of C hydrocarbons was found in the second experiment either, nor was the operation of the regenerator, the stripper, and the reactor noticeably affected. In this case also the butene- 2-lisobutene/butene-1 ratio was substantially increased.

In this example the isomerization was effected in a laboratory unit operating on the same principle and in which the C fraction was injected into an elongated stripper riser in which a temperature of about 482 C. prevailed. A butene-2-l-isobutene/butene-1 ratio of 5.55 was obtained. (Experiment 3.)

In another experiment for comparison the same feed was introduced into the fluid bed in the stripping zone. Further data and the composition of the starting material and the two products obtained in these two experiments are listed in the following Table 3.

Table 11] Starting Recovered (J -fraction material in expet'iin experiment 3 ment 4 isobutane percent by weight" 7. 5 11.7 23.7 n-butane .do 41.1 45. 7 52.3 n-butene- 18. 5 6. 5 4. 1 isobutene 16. 2 15. 5 9. 2 n-butene-2 do- 15. 5 20. 6 10. 7 butadiene do 1. 2 0.0 O. 0

butene-Z-l-isobutene ratio l. 71 5. 55 4.

butene-l We claim as our invention:

1. In the catalytic cracking of a hydrocarbon oil in a fluid catalytic cracking system with a silica-alumina containing catalyst and wherein the used catalyst is continuously cycled to a regeneration zone after stripping it of occluded hydrocarbons with steam in a stripping zone, the improvement which comprises stripping said used catalyst in said stripping zone first with a mixture of steam and an olefinic C -C hydrocarbon fraction containing less than the equilibrium ratio of internal olefins to terminal olefins and then with steam alone whereby there is obtained a stripped product having an increased ratio of internal to terminal olefins and at same time the catalyst is stripped for regeneration.

2. The process according to claim 1 in which the strip ping with the mixture of steam and olefin is efiFected in a riser with the catalyst dispersed in the mixture.

3. Process according to claim 1 in which the said olefinic C -C hydrocarbon fraction contains a ratio of internal to terminal olefins below 2.

4. Process according to claim 1 in which the said olefinic hydrocarbon fraction is a butene-butylene fraction in which butene-l constitutes more than one-half of the sum of butene-2 and isobutene.

References Cited in the file of this patent UNITED STATES PATENTS 2,422,262 Russell June 17, 1947 2,495,648 Voge et al June 24, 1950 2,743,998 Swart et a1. May 1, 1956 OTHER REFERENCES The Chemical Constituents of Petroleum, Sachanen, Rembold Pub. Co. 1945, page 30. 

1. IN THE CATALYTIC CRACKING OF A HYDROCARBON OIL IN A FLUID CATALYTIC CRACKING SYSTEM WITH A SILICA-ALUMINA CONTAINING CATALYST AND WHEREIN THE USED CATALYST IS CONTINUOUSLY CYCLED TO A REGENERATION ZONE AFTER STRIPPING IT OF OCCULAUDED HYDROCARBONS WITH STEAM IN A STRIPPING ZONE, THE IMPROVEMENT WHICH COMPRISES STRIPPING SAID USED CATALYST IN SAID STRIPPING ZONE FIRST WITH A MIXTURE OF STEAM AND 